Recent flybys by Cassini as well as the touchdown of its Huygens probe in January 2005 found a dearth of liquid at lower latitudes, though plenty of channels where it might have run. Instead, vast dunes of sandlike grains composed of hydrocarbons dominate Titan's equatorial region. Yet, dense layers of smog--comprising methane and nitrogen--swath the moon. Methane's continued presence in Titan's atmosphere is puzzling, because sunlight eventually breaks it down to ethane. "Either we are witnessing a very fortunate time in Titan's history," notes Ralph Lorenz, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory, "or the methane is buffered somehow."
This led some, including the late scientist Carl Sagan, to argue for a globe-encompassing sea of methane, or the ethane that results from it. No such sea was discovered, of course, leaving the mystery of Titan's methane unresolved. Now there seems to be evidence for at least some liquid on the moon's surface. "There may be a seasonal cycle in which methane accumulates and evaporates," Lorenz says. "Or it may be that [the lakes] are stagnant."
The Cassini spacecraft used radar to capture images of the lakes while passing the moon last July 22, sending radar information from a stretch of terrain nearly 4,000 miles long and more than 80 miles across. Within this broad area at least 75 dark patches appeared, ranging from under two miles to more than 40 miles wide. "The radar properties of the lakes are consistent with them either being liquids or deposits of very smooth, unconsolidated material: fine particles like very loose soil," Stofan explains. "The material fills topographic lows, some have apparent river channels leading into them and, unlike other areas of Titan, there are no dunes in this area, suggesting a lack of loose, fine material. All of this data together convinced us that these are fluid-filled lakes."
It is not clear exactly what fluid fills the lakes--most likely some combination of methane and ethane--or how they are filled--either through a weather system of methane evaporation from lake surfaces followed by hydrocarbon rain or via liquid stored below the surface (a "methanifer" rather than an aquifer). "All of these--the methanifer, the lakes, the methane in the atmosphere--are part of an active cycle that is likely to be varying seasonally," Stofan adds. "All are interacting with each other to create a dynamic system--something we have not seen on any planet other than Earth."
Future flybys in coming months and years should reveal how the lakes vary seasonally, as northern Titan progresses from its long, dark winter into summer, as well as whether lakes exist elsewhere on its surface. A similar object--dubbed Lake Ontario for its size and shape--was seen near the summery southern pole during earlier observations and so far only 15 percent of the moon's surface has been mapped. But it will take a mission dedicated specifically to exploring Titan--perhaps by balloon--to determine the nature and extent of these lakes. "Titan has long been thought to be a world with all of the building blocks of life, with what is referred to as prebiotic chemistry taking place on the surface," Stofan says. "We would love to know if things like amino acids have formed in or near these lakes."
"Titan is more interesting than we thought," Lorenz adds. "There are very few things that we thought could be possible that Titan hasn't graced us with." The paper presenting the finding is published in the January 4 Nature.